This invention relates in general to a method for compensating an output signal of an inertial sensor to correct for varied sensor mounting locations. In particular, this invention relates to a method for compensating accelerometer output signals with other vehicle sensor outputs for use in a vehicle stability control system.
Performing vehicle stability control with an electronic control unit (ECU) requires accurate inputs of a vehicle's inertial state. This is accomplished with one or more angular rate sensors for detecting and measuring vehicle rotation about a vehicle spatial axis and one or more of a lateral, longitudinal, and vertical accelerometer adapted to measure a vehicle inertial state in a corresponding axis. The vehicle spatial axes are those axes having a point of origin at a vehicle center of gravity, CG. For example, a roll rate sensor may be provided for measuring an angular velocity about a longitudinal axis, and either a single axial accelerometer, such as a lateral accelerometer, or multiple accelerometers, such as a lateral accelerometer and a longitudinal accelerometer, may be provided to measure the corresponding axial acceleration. Regardless of how many accelerometers are utilized, it is necessary that all of the accelerometers are accurately mounted in a known relationship to the vehicle's three spatial axes and the relative rotational degrees of freedom about each spatial axis. The mounting accuracy of the accelerometer relates to the co-linearity of a principal measurement or sensing axis of the accelerometer to the corresponding vehicle spatial axis.
The three vehicle spatial axes for a vehicle 10 are illustrated in FIGS. 1 and 3 where the axis labeled 12 corresponds to the vehicle longitudinal axis that runs along the length of the vehicle and points toward the forward direction of movement for the vehicle. A second spatial axis labeled 14 corresponds to the vehicle lateral axis and is perpendicular to the longitudinal axis 12. A third spatial axis labeled 16, that extends perpendicular to the plane formed by the longitudinal and lateral axes 12 and 14, corresponds to a vertical axis of the vehicle. The three vehicle spatial axes intersect at the vehicle center of gravity (CG) 11. Thus, an accelerometer for measuring acceleration of the vehicle would be ideally mounted at the vehicle CG.
The three relative rotational velocities are also illustrated in FIG. 1 where the circular arrow labeled 18 that is centered upon the longitudinal axis 12 corresponds to vehicle roll velocity while the circular arrow labeled 20 that is centered upon lateral axis 14 corresponds to vehicle pitch velocity. Similarly, the circular arrow labeled 22 that is centered upon the vertical axis 16 corresponds to yaw velocity. Each of these rotational velocities may be measured by a rotational velocity sensor, or angular rate sensor, that would ideally have its axis of rotation parallel to the vehicle spatial axis about which the rotational velocity occurs. Thus, for a measurement of roll velocity, the corresponding roll velocity sensor would have its axis aligned with the vehicle longitudinal spatial axis 12.
Ideally, the rotational velocity sensors and accelerometers are each mounted with their principle sensing axes aligned with one of the vehicle's corresponding three spatial axes. If the rotational rate sensors and accelerometers are not accurately mounted, erroneous information will be transferred to the ECU. A method to correct for mounting offset errors of sensors having sensor axes that are substantially coincident with the vehicle's three spatial axes is disclosed in U.S. patent application Ser. No. 11/712,321, filed Feb. 28, 2007, the disclosure of which is incorporated herein by reference in entirety. The method for correcting sensor mounting errors relative to the vehicle spatial axes is a function of the angular misalignment of the sensor axis to the corresponding vehicle spatial axis. The sensor output correction is in the form of a calibration factor that mathematically realigns the corresponding sensing and vehicle spatial axes.
However, because of the wide variety of vehicle platform architectures that are manufactured, it is not always practical to mount the accelerometers in a coincident orientation to the vehicle CG, or even each of the three vehicle spatial axes. Instead, it often becomes necessary to provide an off-axis mounting location for an accelerometer where at least one of the vehicle spatial axes does not intersect with the inertial sensor mounting location. It is therefore desirable to facilitate a higher degree of flexibility in sensor mounting locations within the vehicle platform. In addition to compensating for sensor mounting inaccuracies, it would be desirable to provide a correction method to compensate for inertial sensors mounted in an off-axis location. It would be further desirable to utilize existing subsystem rotational, acceleration, and angular velocity sensors as inputs into a dynamic correction algorithm.